Cosmetic problems typically point to flow restrictions.
I received a package from one of my old friends the other day. The package contained many small lenses, each one showing a different problem with cosmetic issues at the gate. The small cold runner that accompanied the parts was a classic example of what not to do when designing a gate to feed a part that requires high clarity and no blemishes. (This design flaw is one that I show on the Gate Dimensioning page in one of my seminar books)
Tab tricks
Any time you have a tab attached to a part, you need to be careful about three things.
First, be sure you are gating into the tab from the side, not the top. This is so the material flowing in through the gate will swirl around in the tab prior to entering the main body of the part and will basically eliminate any tendencies for blush or streaks.
Second, the tab thickness needs to be the same as the part wall thickness. This is done so the tab is really just an extension of the part. Third, be sure the depth of the gate feeding into the tab is the same depth as the one feeding directly into a part wall. In this case, the material is acrylic, so the gate depth would be equal to 75% of the tab thickness.
This tab was .130 inch thick. Based on the 75% rule for acrylic, the gate depth feeding into this tab should have been .100 inch. This sizing rule is used to avoid flow restrictions, which might lead to blush or streaking in the tab or the part.
Remember, when designing a gate to feed a tab, the gate land length should be .030 inch or less and the gate width should be no wider than the diameter of the runner feeding it. The volume of material required to feed all the mold cavities determines the width: The more volume, the wider the gate.
Another issue jumped out at me while I was looking at the tab gating: the diameter of the sprue bushing and the nozzle orifice. The existing sprue O-diameter was .220 inch, fed by a .125-inch nozzle orifice. With a .250-inch runner diameter, it didn’t take much to see the sprue bushing and nozzle orifice were quite a bit undersized.
The nozzle orifice should be 10% smaller in diameter than the sprue O-diameter. This is done to prevent the sprue from hanging up between shots in case the injection unit is not aligned correctly or if the nozzle gets bent slightly from excessive heat being applied to it.
For a runner of .250 inch, we needed a sprue bushing with an O-diameter of .312 inch (about 10-25% larger). The nozzle should have been a full taper for an amorphous material such as this acrylic. The nozzle orifice needed to be .290 inch to feed the new .312-inch sprue bushing. This would allow the barrel heats to be reduced from where they were, and possibly the cycle time could be cut by 25% or so.
This would generate a big change in flow. When we increase a runner diameter or a nozzle orifice by 50%, we double the flow of material through that area. By increasing the nozzle orifice from .125 inch to .290 inch, we’d more than double the flow of material through the existing nozzle, which means we’d more than quadruple the flow of material into the mold cavities. This means we could fill and pack the part without using as much heat and without as much effort or pressure to get the job done.
Side Benefits
We figured out the cause of the defects by looking at the examples in my seminar book. We also corrected a flow problem the customer might not have known existed. Think of it: We can use less heat to get the acrylic to fill and pack the parts, and maybe even run a faster cycle.
What does this mean to the molder? Well, for one thing, it means he can get more passes out of the acrylic before the regrind starts to change to a slightly tinted shade. It also means he can switch to a different grade of acrylic to mold these parts, knowing that some of the stiffer grades are usually more scratch resistant than the easier-flow grades. What a great opportunity not only to eliminate the reject issue first indicated, but also to improve the properties of the part being molded!
This was not an especially difficult problem this month, but for those that run parts with cosmetic requirements, it should prove useful in correcting persistent problems or designing new tools to run cosmetic parts, such as these lenses.
I called the molder with my design suggestions. His response was, “Where can I find this kind of information on my own?” All I could think to say was, “I just haven’t gotten around to writing the book yet.”
Maybe writing books on subjects such as these will keep me busy when I retire. Then I wonder if I will ever retire. I am having too much fun doing what I do to sit around the house clipping coupons.
The Troubleshooter’s Notebook
Part/material:
Small acrylic lenses
Tool:
Single-cavity, cold runner
Symptoms/problem:
Cosmetic blemishes at the gate
Solution:
Increase sprue bushing and nozzle orifice diameters; change to a full-taper nozzle; gate tab from the side instead of the top; increase tab thickness to the same thickness of the part; increase tab gate depth
Gate Dimensioning
These are the tips Bob Hatch uses in his seminars for determining proper gate dimensions:
1. Gate depth controls freeze-off only.
Gate depth for easy-flow PE and PP: 50% of the thickness of the wall being gated into.
Gate depth for ABS, SAN, and acrylics: 75% of the thickness of the wall being gated into.
Gate depth for PC, acetal, PVC, glass-filled materials, and PPO: 90% of the thickness of the wall being gated into.
The volume of material that flows through the gate during injection should be the same as the volume that flows through an edge gate that is twice as wide as it is deep. Use the area of the gate, in square inches, as a guide to modifications. For example, an edge gate of .050 by .100 inch would equal a subgate of .071 inch in diameter.
2. Land length works in conjunction with the gate depth.
Land should equal half the depth, but never exceed .030 inch for any material.
You can use .020 inch for PE, PP, and unfilled nylons.
3. Gate width controls the filling of the part only.
Width normally equals two to three times the gate depth.
If your part is larger or thicker than normal (generally .075-.150 inch thick), try making the width of the gate five to 10 times the gate depth. Increase the diameter of the runner in this case.
Trial and error is the only way to determine the correct gate width.
It’s counterproductive to make the width wider than the diameter of the subrunner that feeds the gate. You’ll end up losing pressure through the gate.
About
Bob Hatch Bob Hatch is one of the leading on-the-spot problem solvers in the molding industry. Mr. Hatch spent time as the technical programs manager at Channel Prime Alliance and managed a molding operation for more than 25 years. Currently, he writes articles for Injection Molding Magazine under the pseudonym The Troubleshooter.
